Stabilization of acrylamide



2,921,942 STABILIZATION F"ACRYI .AMlDE Norbert M. Bikales, Stamford, andRobert-J. Munch, Greenwich, Conn., assignors to American CyanamidCompany, New York,- N.Y., a corporation ofMaine No Drawing. ApplicationMay 21, 1956 Serial No. 585,936

2 Claims.- .(Cl. 260-5561) 'This invention relates to unsaturatedorganic amides. More particularly, it relates to acrylamide and its'a-substituted homologs and particularly to the stabilization ofsolutions of such amides against polymerization.

Acrylamide and its tat-substituted homologs and-the polymerization andcopolymerization products thereof areemployed in various fields.Especially are they of importance in such fields as adhesives,dispersants, plasticizers, surface coatings, thickening agents as wellas in the leather, paper, textile, and photographic arts. Because ofthese many potential applications, methods of preparing the monomers areof particular interest. a

These monomers are usually prepared by reacting the correspondingacrylonitrile with water and concentrated sulfuric acid. The reactionmixture comprising the amide sulfate is then furthertreated to separatethe free amide. This may be conducted in various ways, one of whichcomprises treating an aqueous solution of the amide sulfate with analkaline earth metal hydroxide or carbonate to precipitate an insolublesulfate. Another procedure comprises treating a solution of the amidesulfate in an organic solvent with ammonia or an alkaline metalhydroxide or carbonate to form the corresponding sulfate which isinsoluble in the solvent. An unusually successful procedure particularlyadapted for the neutralization of acrylamide sulfate comprises treatingan aqueous solution thereof with ammonia under controlled conditions oftemperature and hydrogen ion concentration. Under these controlledconditions, the normally watersoluble ammonium sulfate precipitates andmay be sep arated.

In each of these modes of neutralization, the residual liquor is usuallytreated to recover a crystalline product. Regardless of how theneutralization and crystallization are conducted, elevated temperaturesare generally employed at some point in the process which aggravate theproblem of premature polymerization existing because the amide is insolution. This problem is still further aggravated in continuousneutralization systems because of the extended residence time within thesystem of part of the original liquor due to the recycling of amide-beaning streams.

Although the present invention is particularly concerned with theproblem of preparation of acrylamide and its homologs, the problem alsoexists on other situations. Thus, polymerization is also a seriousproblem in those instances in which it may be desired to store asolution of an amide. Similarly, the problem exists when the amide isemployed as an intermediate under conditions favorable forpolymerization. It is apparent, therefore, that there are varioussituations in which stabilization of an amide solution is highlydesirable.

It would appear that this problem of premature polymerization could bereadily overcome by incorporating in the amide-bearing solution any ofknown materials which have been suggested as polymerization inhibitors.

Many of these materials, however, have been found unpolymerizationduring the satisfactory for one or more of various reasons. Some 1 7extended period of time."

satisfactorily inhibit polymerization but discolor the solution and/orare diflicult to remove prior'to use of the solution for polymerization.Still others have been found to be ineffective as polymerizationinhibitors for any In accordance with the present invention, it has nowbeen found that the polymerization of solutions of acrylamide and itsd-SllbStitlltCd homologs can be effectively inhibited for extendedperiods of time by incorporating therein a material represented by theformula:

in which R 'is analkyl radical of l to 4 carbon atoms and n'is lor 2.

The inhibitors of this. invention arefreadily prepared by reacting theappropriatesecondary amine with carbon bisulfide. The resultantdithiocarbamate may then be converted into the corresponding monoordisulfide. The compounds forming the present class of polymerizationinhibitors are known and several have had fairly extensive use as ultraaccelerators in the vulcanization of rubber for a number of years. Thesurprising effectiveness of these compounds as polymerization inhibitorsfor acrylamide and its tit-substituted homologs, therefore, is quiteunexpected.

The amotmt of inhibitor employed may vary over a wide range. It has beenobserved that stabilization is considerably enhanced by employing aslittle as 10 parts of inhibitor per million parts of amide solution.Usually, however, the amount employed will be greater than this and itmay even range to as high as 2,000 p.p.m. Beyond this concentration,however, there is little if any additional stabilization advantagegained. It is a preferred practice in accordance with this invention toemploy the inhibitor within a range of about -1000 Aside from exhibitingan unusually effective stabilizing property, inhibitors of thisinvention have the further advantage in that they do not discolor theamide product and may be easily separated when desired. The inhibitormay be readily removed from the amide product by solvent extraction.

The invention is further illustrated by the following examples. Unlessotherwise noted all parts are by weight. 1

EXAMPLE .1

Table 1 Sample Tetramethylthluram 0 10...-- 100--.- 500--.. 1,000.

monosulfide (p.p.m.) Polymerization time 22 min 18 hr" 43hr.- 66hr 62hr.

inhibitors added as shown in Table 11- 3 EXAMPLE 2 A 73% solution ofacrylamide in water containing 1.3% ammonium sulfate is divided into 3samples and The samples r then placed in sealed tubes and maintain d atwhile observing the fimefor polymerization to set in. Results appear inTable ,II.

EXAMPLE 3 The procedure of Example 1 is repeated except that a 60%solution of methacrylamide is employed. Substantially similarstabilization results are obtained.

Extended stabilization is similarly obtained with the other inhibitors 7not this invention.

We claim:

1. A method of inhibiting the polymerization of an amide selected fromthe group consisting of acrylamide and lower ot-alkyl acrylamides whichcomprises mixing therewith at least about 10 parts per million oftetramethylthiuram monosulfide.

' 2. An amide selected from the group consisting of acrylamide and loweran.- alkyl acrylagnigles vstabilized against polymerization containingas a polymerization inhibitor tetramethylthiuram monosulfide.

References Cited in the file of this patent UNITED STATES PATENTSAntlfiinger Dec. 15, 1953 Breslow June 5, 1956 OTHER REFERENCESKluehesky et -al.: Ind. and Eng. ,Chemistry, vol. 41, N0. 8, p. 1768.

Ferington et al.: Journal of the American Chemical Society, vol. 77, p.4510-4512 .(1955).

Dunbropk: India Rubber World, .vol. 117,-pp. 485 and 552 1948).

A eli Fundam n ri ci e o o ymetiz iqn (19 2) John W ey d 3 41 In New i.P-

1. A METHOD OF INHIBITING THE POLYMERIZATION OF AN AMIDE SELECTED FROMTHE GROUP CONSISTING OF ACRYLAMIDE AND LOWER A-ALKYL ACRYLAMIDES WHICHCOMPRISES MIXING THEREWITH AT LEAST ABOUT 10 PARTS PER MILLION OFTETRAMETHYLTHIURAM MONOSULFIDE.